For example, in the related art, PTL 1 discloses a boiler in which water is heated by an economizer using a flue gas, and saturated steam generated from a drum is turned into superheated steam by a superheater and supplied to a steam turbine. In addition, in order to cope with a case where the temperature of the superheated steam supplied to the steam turbine is high, PTL 1 discloses a spray device which, using water before being supplied to the economizer as a coolant, mixes the coolant with the superheated steam and cools the superheated steam at the outlet of the boiler so as to control the temperature of the superheated steam to a predetermined temperature.